How to request retraction of a published paper when contacting the editors and publishers doesn't work?

If you have tried the journal/publisher's complaint process without success, and you believe that the journal editors are not handling the matter according to commonly accepted publication ethics standards, you can contact COPE, the Committee on Publication Ethics. All Elsevier journals are members of COPE.

If a reader or an author has concerns about a member editor, journal or publisher which they wish to raise to the attention of COPE, they should email the COPE Membership Assistant and supply the following information:

• A summary of the concerns in 500 words—this will be shared with the editor and/or publisher and thus the summary should be as clear as possible and provided in a polite tone.
• An outline of the points within the COPE Code of Conduct which are relevant to the concerns.

Source: COPE: Contact Us

This flowchart explains how COPE handles complaints about member journals.

As others have mentioned, contacting the editors of the journal (and authors of the paper) is the recommended first step. However, since that didn't work ...

You can always take a "name and shame" approach.

One possible step is to write an official "letter to the editor". That is, not just a short unofficial, back-channel email to the editors, but a full on official letter of the form that would be typically published in the front part of the journal. This would indicate to the editors that you're serious about this issue, and are willing to stake your name and reputation on the accusations of plagiarism. -- Keep in mind, though, that the editors may decide not to publish your letter.

If the journal itself is not willing to be an outlet for informing readers about the plagiarism, you can seek alternative venues for it. Other journals are unlikely to be interested, but social media might be a possibility. I don't know about Elsevier, but many companies monitor places like Twitter, and respond in a more timely manner to issues raised there. If you post the information there (with the appropriate "@ mentions" to bring it to the journal's and Elsevier's attention) you may get more traction - particularly if you mention that approaching it through the standard channels didn't work. Again, you will want to do this "officially", from a Twitter account associated with your academic presence, rather than from a recreational, pseudonymous account.

There are also social media accounts and organizations which are interested in misconduct in journal publishing, and they may be interested in helping you pursue this, or at the least serve as a platform to inform people of the issue. Retraction Watch may be an option, particularly if you can demonstrate that the journal in question is stonewalling you and the author(s) in pursuing a retraction. (They can also serve as a neutral third party to let you know if the issue is compelling/serious as you think it is, so this might be a good first choice.)

As final note, I'd caution you to be careful in pursuing a "name and shame" approach. As I alluded to, it works best if you are willing to put your name and academic reputation behind the accusations of plagiarism. Be sure you can adequately demonstrate evidence for all of the accusations (of plagiarism or stonewalling) before making them.

Also keep in mind that there may be a difference of opinion here. The editors are not going to retract the paper just on your say-so. Journal-originated retraction, or retraction when not all the authors agree to it is a serious deal, and the editors will be (rightfully) hesitant to do it. The editors may err on the side of caution, and believe that the evidence of plagiarism is not as compelling as you make it out to be.

If you work in mathematics, the European Mathematical Society has an Ethics Committee to who you can present your case. They should contact the editor and Elsevier, hopefully with more traction than you had, and can issue public statements if there is no satisfying resolution. This could help you substantiate your claims whenever you need to, and also should harm the offenders.

If you don't work in mathematics, I suggest you look whether any learned society in your field has a similar ethic committee you could appeal to. Note that the EMS ethics committee considers cases from any mathematician, not only European ones.